A disc drive storage system 300 of FIG. 5 stores data for use by computer systems and electronic products that require internal data storage. The system 300 comprises a disc 312 on which is deposited magnetic material for storing information in the form of magnetized domains having a magnetized state representing either a binary one or a binary zero. The information is written to the disc 312 by magnetizing the domains during a write operation. The domains retain the magnetization for later retrieval during a read operation. The magnetized state is determined and the stored information derived therefrom for use by the computer system or electronic product.
The disk drive 300 comprises a magnetic recording medium in the form of the disk or platter 312 having a hub 313 and a magnetic read/write transducer 314, commonly referred to as a read/write head, for reading data stored on the disk 312 and writing (storing) data to the disk 312. The read/write head 314 is attached to or formed integrally with a suspension arm 315 suspended over the disk 312 and affixed to a rotary actuator arm 316. The actuator arm 316 is pivotably connected to a platform 320 at a pivot joint 322. A voice coil motor 324 drives the actuator arm 316 to position the head 314 over a selected location on the disk 312. A surface of the disk 312 is divided into a plurality of concentric tracks 326, each track comprising user data fields (including error correction coding bytes), servo tracking fields and timing/synchronization fields.
Although only a single disk 312 is illustrated in FIG. 5, a conventional disk drive system comprises a plurality of double-sided disks oriented in a stacked configuration, with one head servicing one side of each disk.
In other data storage systems the head 314 operates with different types of storage media (not shown in the Figures) comprising, for example, a rigid magnetic disk, a flexible magnetic disk, magnetic tape and a magneto-optical disk
As shown in a partial cross-sectional and partial block diagram of FIG. 6, the disk 312 comprises a substrate 350 and a thin film 352 disposed thereover. The head 314 comprises a write head 314A and a read head 314B.
Data bits to be written to the disk 312 are supplied by a data processing device 360 (e.g. a computer or music player) to a data write circuit 362 where the data bits are formatted and error detection/correction information appended thereto.
To write data bits to the disk 312, the voice coil motor 318 moves the suspension arm 316 to a desired radial position above the surface of the disk 312 while the spindle motor rotates the disk 312 to move a circumferential track region to be written under the write head 314A. A write driver 364 responsive to the data write circuit 362, scales up the relatively low voltages representing the data bits to a voltage range between about +/−6V and +/−10V and supplies a write current (typically between about 10 mA and 70 mA) to the inductive write head 314A. The write driver 364 also shapes the write current signal waveform to optimize the data writing process. The write driver 364 is conventionally an element of a preamplifier 366, and in one embodiment the preamplifier 366 comprises an element of an electronics module 330 (see FIG. 5) connected to the head 314 via conductors 332.
Write current supplied by the write driver 364 to the write head 314A (magnetically coupled to a magnetically permeable core not shown) creates a magnetic field that extends from the core across an air gap between the write head 314A and the disk 312. The magnetic field alters ferromagnetic domains in the thin film 352 to store the data bits as magnetic transitions.
The direction of the magnetic field generated by the write head 314A, and thus the direction of the altered ferromagnetic domains, is responsive to the direction of current flow through the write head 314A. Write current supplied in a first direction through the write head 314A causes the domains to align in a first direction (representing a date bit 0 for example) and write current supplied in a second direction (representing a data bit 1 for example) causes the domains to align in a second direction.
In the read mode the magnetic field of the ferromagnetic domains is detected to determine the stored data bit. The read head 314B (comprising a magneto-resistive (MR) sensor) senses the magnetic field transitions in the thin film 352 to detect the stored data bits. State-of-the-art MR read heads include giant magnetoresistive (GMR) heads and tunneling magnetoresistive (TMR) heads.
To read the data bits, the suspension arm 316 moves the head 314 while the disk 312 rotates to position the read head 314B above a magnetized region to be read. A read circuit 368 of the preamplifier 366 supplies a DC (direct current) bias voltage of between about 0.025V and about 0.2V across the read head 314B. Alternatively, the read circuit 368 can provide a controlled bias current ranging typically from about 50 uA to 5 mA to the head 314B. The bias circuits regulate voltage or current head bias only at low frequencies, and present a high impedance to the head 314B at mid- and high-frequencies, thus permitting mid- and high-frequency read data to be sensed across the head.
The magnetic field of the ferromagnetic domains in the thin film 352 passing under the read head 314B alters a resistance of the magneto-resistive material, imposing a differential AC (alternating current) component on the DC bias voltage. This bias voltage (or current) ensures that the head 314B operates in a linear response region, i.e., the resistance varies linearly responsive to the sensed magnetic field. The AC component representing the read data bits has a relatively small magnitude (e.g., a millivolt) with respect to the DC bias voltage.
The differential signal from the read head 314B is amplified in the read circuit 368. To reduce noise effects in subsequent signal processing stages, it is desired to maximize the amplification (gain) of the read circuit 368 consistent with signal linearity requirements and available power. Thus a first stage of the read circuit 368 typically comprises a low noise amplifier. The amplified signal is input to a signal processing stage 402 to further amplify the differential signal. The scaled-up signal is supplied to a channel chip 406 where data-detection (preferably using partial-response maximum-likelihood, or iterative decoding, techniques), error detection and correction processes are performed to detect the data bits from the voltage generated by the head 314B. The read data bits are returned to the processing device 360 via a user interface 410 (e.g., SATA, SCSI, SAS, PCMCIA interfaces).
Disk drive manufacturers and manufacturers of systems employing disk drives have an interest in knowing a read head resistance Rmr, i.e., the resistance of the MR sensor. Generally, the head resistance ranges from about 20-600Ω. Manufacturing tolerances among heads of the same material and construction can vary substantially, by several percent decades. The head resistance can also be affected by aging, heat and long-term electromigration in the head material. An optimum read head bias is related to the head resistance, and thus knowing the head resistance permits the disk drive manufacturer to employ the optimum bias.
To switch the head operation from writing to reading, the writer circuits are deactivated and ideally the read head is immediately ready to read the disk. However, the servo control loops in the read circuit 368 that supply the bias require a finite time to reach a steady-state condition. In particular, the components supplying the bias must be permitted sufficient time to ramp up from a zero DC bias to a desired steady state bias (referred to as a loop settling response time), without significant overshoot. State-of-the-art MR bias-control loops respond in about 50 ns. In certain applications for the disk drive system, it is required to bias the read head to within about 3% of its bias tolerance with a 50 ns settling time. As is known, the loop response characteristics are a sensitive function of loop bandwidth and gain, and the loop gain is in turn a function of the head resistance. It may therefore be difficult to stabilize the transient loop characteristics to avoid overshoot and undershoot over the entire expected head resistance range of 20-600 ohms within the desired settling time. If the value of Rmr is known, an optimal loop gain can be established irrespective of changes in Rmr and the loop settling time thereby minimized.
Non-optimal loop settling time may also require the manufacturer to allocate valuable track data storage space to dead zones, thus reducing storage capacity. For example, read only servo bursts for use in head control, are interspersed with readable/writable data records on the disk. When a write operation is complete a transition must be made to the read mode to read the disk servo bursts. If write-to-read recovery is long, the write operation must be terminated farther ahead of the servo data bits on the disk than would be necessary for a short write-to-read recovery time. The disk area covered by the head during the transition time cannot be used to store user data and is therefore referred to as a dead zone. The disk drive system designer always budgets for a worst-case or slow recovery when designing a disc format, and that format must include the dead zones.
In addition to minimizing loop settling time, proper utilization of the known Rmr can benefit other aspects of disk drive operation. For example, in certain implementations of the read circuit 368, head bias current or voltage is supplied through source- or emitter-followers through ballast resistors that are large relative to the head resistance and thus minimize loading across the head. Knowledge of the head resistance permits selection of optimum values for these ballast resistors, e.g., a value to minimize the head noise figure. Determining the head resistance can also identify a failed head, as resistance values exceeding a critical value RmrMAX generally indicate a gross head failure.
As can be seen, knowledge of the Rmr value is advantageous for optimum operation of the read circuit and for optimal performance of the disk drive system. Once the head resistance is known, operational parameters of the read circuit can be established to optimize performance, including bias loop gain and bandwidth (which impact the bias loop transient response) and noise performance
One approach to dealing with head resistance variation relies on designing the read circuit based on a nominal expected MR head value; however, manufacturing variations and a high sensitivity to resistance variations can lead to unacceptable performance variation when this approach is employed. Therefore, it is preferred to measure the resistance of each head in a disk drive system. Current read circuits (the read circuit 368 in FIG. 6) for use with an MR read head are designed to operate over one or more head resistance subranges within a total expected resistance range. An exemplary read circuit operates over one or more of the following selectable resistance subranges: 20-90 ohms, 50-250 ohms, and 100-400 ohms. Most read circuits are not capable of operating over the entire range of 20-400 ohms. The disk drive system manufacturer selects one of the ranges based on an expected head resistance and the reader circuits operate accordingly, even if the read head resistance is outside the selected range. Thus the selected read circuit range imposes strict tolerance requirements on the head resistance, and the coarseness of the selectable Rmr ranges precludes fine optimization of the settling response of the read circuit control loops. Certain disk drive manufacturers desire a uniform settling response over at least a two octave head resistance span, thus further aggravating settling time issues. Clearly, simply selecting an Rmr range for the read circuit does not provide optimum read circuit operation, especially if the head resistance Rmr changes with time.
Alternatively, to refine settling response time and other reader circuit parameters, the disk drive manufacturer can include within the read circuit of the preamplifier the necessary hardware components to measure the head resistance, when used in conjunction with special-purpose software operating elsewhere in the disk drive. Although existing preamplifiers may include the hardware features for making this measurement, there is some reluctance among disk drive manufacturers to measure Rmr dynamically using a software approach. The burdens of writing and certifying new microcode, the unavailability of existing data structures in which to store/recall the Rmr measurement results and modification of well-established production flow procedures to implement new code are cited as reasons for this reluctance.
To measure the head resistance it is also known, for example as disclosed in U.S. Pat. No. 6,225,802 to Ramalho et al. to sequentially and automatically supply different current values to the head until the head voltage equals a reference voltage. The resistance can then be determined from the known voltage and current. This technique consumes significant power when determining Rmr and consumes silicon area when implemented in the integrated circuit comprising the preamplifier, since it cannot make dual use of significant reader circuitry used in normal reader operation. The method is also time consuming in that the test current values are supplied sequentially, thereby protracting the time until the head is ready to read the disk. This method also does not preserve the common mode voltage on the head, which can lead to sensitivity to momentary head-disc contact or, in extreme cases, to electrical breakdown of the air-film bearing on which the head flies. Further, the results obtained are susceptible to corruption should a thermal asperity coincide with the measurement process. Ramalho discloses use of the measured MR resistance to optimize head signal-output level or to post a fault tag whenever measured resistance falls outside a predetermined range. He does not disclose the advantages to read circuit settle-time performance that can be obtained by use of the MR resistance measurement to specify the loop-gain of MR bias-control loops